Rapid improvement of portable electronic devices, such as cellphones and laptops, leads to requirements for higher capacity of batteries used for main power supplies and uninterruptible power supplies of such portable devices. Then, electrochemical devices which are non-aqueous electrolyte batteries, such as lithium ion secondary batteries, have gained attention because such batteries have a higher energy density than nickel-cadmium batteries and nickel-hydrogen batteries.
Typical examples of the electrolyte solution for lithium ion secondary batteries include non-aqueous electrolyte solutions prepared by dissolving an electrolyte (e.g., LiPF6, LiBF4, LiN(CF3SO2)2, LiCF3(CF2)3SO3) in a solvent mixture of a high-dielectric-constant solvent (e.g., ethylene carbonate, propylene carbonate) and a low-viscosity solvent (e.g., dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate).
Major negative electrode active materials for lithium ion secondary batteries are carbonaceous materials that can occlude and release lithium ions, and typical examples thereof include natural graphite, artificial graphite, and amorphous carbon. Further, metal- or alloy-based negative electrodes containing silicon, tin, or other elements are also known to provide a much higher capacity. Major positive electrode active materials for the above batteries are transition metal complex oxides that can occlude and release lithium ions. Typical examples of the transition metals include cobalt, nickel, manganese, and iron.
Such lithium ion secondary batteries include highly active positive and negative electrodes. Such electrodes disadvantageously cause side reactions with electrolyte solutions, and these side reactions are known to decrease the charge and discharge capacities. In order to suppress such a disadvantageous battery feature, researchers have performed various studies on non-aqueous solvents and electrolytes.
Patent Literature 1 proposes to use an electrolyte solution containing an organic compound having two or more nitrile groups. The nitrile groups are polarized to give a large dipole moment, and this large dipole moment suppresses oxidative decomposition of the electrolyte solution on the positive electrode during charge at high voltage. Thereby, the above disadvantageous battery feature is suppressed.
Patent Literature 2 discloses an agent for forming films on electrode surfaces. The agent contains a specific nitrile compound, and thus can improve the thermal stability of batteries.
Patent Literature 3 discloses a non-aqueous electrolyte secondary battery that contains a fluorinated nitrile compound in an electrolyte solution and thus has excellent charge and discharge efficiency and storage characteristics.
Patent Literature 4 discloses that addition of a compound having an isocyanate group to a non-aqueous electrolyte solution suppresses a decomposing reaction of a solvent on the negative electrode, and thus improves the cycle characteristics of batteries.
Patent Literature 5 proposes to form a complex of an aliphatic nitrile compound with the surface of a positive electrode active material, and thereby form a protective film on the positive electrode. This improves the safety of batteries against overcharge and/or physical impacts from the outside.
Patent Literature 6 proposes to use a sulfonic acid salt-type compound having at least one substituent selected from the group consisting of a cyano group, an isocyanate group, a thiocyanate group, and an isothiocyanate group so as to improve the life and high-temperature durability of lithium batteries.
Patent Literature 7 proposes to use a sulfuric acid ester-type compound having a C(sp)-C(sp3) unsaturated hydrocarbon bond so as to improve the high-temperature cycle characteristics of lithium batteries.